<!DOCTYPE html>
<html>
    <head>
        <title>Example</title>
            <!-- 
            em--与元素字号挂钩
            ex--与元素字体的‘x高度’挂钩 1ex大致等于0.5em
            rem--与根元素的字号挂钩
            px--css像素(其实用为绝对单位)1px=1/96in
            %--另一属性的值的百分比
         -->
    <style>
        p {
            background: grey;
            color: white;
            font-size: 20px;
            width: 200px;
        }
    </style>
    </head>

    <body>
        <a href="http://apress.com">
            Visit the Apress website
        </a>
        <p>I like <span>apples</span> and oranges.</p>
        <a href="http://w3c.org">Visit the W3C website</a>
    </body>
</html>